1. Field of the Invention
The present invention relates to optical communication including optical regeneration and/or wavelength conversion.
2. Related Art
Optical signals are increasingly relied upon to carry data at high bit rates over long distances in optical fiber transmission systems. For example, digital data is modulated on an optical carrier signal. The digital data is composed of bits (e.g., xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d). Ever higher bit rates are increasingly desired to increase the bandwidth or amount of data that can be carried over a fiber link. Optical carrier signals can have wavelengths in the visible to infrared spectrum. Wavelength-division multiplexing (WDM) fiber transmission systems support multiple optical carrier signals at different wavelengths. For example, two windows where optical carrier signals having wavelengths equal to around 1310 nm and/or 1550 nm and carrying data at bit rates greater than 10 Gigabits/sec are used or proposed for many high-speed WDM optical communication systems.
Optical signals degrade as they travel over long distances in optical fiber transmission systems. Such degradation is due to a number of factors, such as, loss, chromatic dispersion, fiber non-linearities, and other fiber impairments. These impairments reduce the signal-to-noise ratio of the optical signal. This can lead to bit loss and other errors, especially at high bit rates. Temporal as well as spectral content of the optical signal can be distorted. In-line optical amplifiers are used to compensate for loss. However, this amplification introduces optical noise to an optical signal which can accumulate to an unacceptable level when an optical signal has passed through long chains of in-line optical amplifiers.
Regeneration systems are used to xe2x80x9cregeneratexe2x80x9d optical signals. Regeneration can restore or improve aspects of a degraded optical signal. Optical 2R regeneration involves re-shaping and re-amplifying a degraded optical signal. Optical 3R regeneration involves re-shaping, re-amplifying, and re-timing a degraded optical signal.
Conventional optical regeneration techniques include two types: opto-electronic regeneration and all-optical regeneration. Opto-electronic regeneration uses direct detection to convert optical signals to the electrical domain, and then uses re-modulation to convert back to the optical domain. Opto-electronic regeneration is expensive and bandwidth-limited.
In all-optical regeneration, optical signals remain in the optical domain during regeneration. There is no need for conversion to the electrical domain. Also, all-optical regeneration of degraded data will be crucial for future large scale photonic networks with channels transmitted at variable distances and resulting in signal quality discrepancies between channels (Simon, J., et al., ECOC ""98:467-469 (Sept. 1998)).
Three conventional all-optical regeneration techniques, however, also have drawbacks. First, fiber-based optical regeneration systems are bulky, increase latency, non-integrable, and environment-sensitive. For instance, a recent reported technique of all-optical regeneration at 40 Gb/s using Kerr effect in fibers showed an impressive performance (Pender W., et al, Electron. Lett. 32 (6): 567-569 (March 1996)). However, the operation of a Kerr regenerator requires a long length of fiber.
Second, regenerators that use semiconductor optical amplifiers in an interferometer add broadband optical noise (ASE noise) and are expensive to fabricate. Their speed is limited by the carrier recombination rate. The sinusoidal transfer function of semiconductor optical amplifiers also gives rise to weak thresholding, thereby limiting the signal-to-noise quality of the re-generated optical signal. For example, one interferometric wavelength converter showed impressive performances, but required complex fabrication steps. See, Wolfson, D., et al., xe2x80x9cAll Optical 2R regeneration at 40 Gb/s in an SOA-based Mach-Zehnder interferometer,xe2x80x9d in OFC ""99, PD 36, San Diego, Calif. (February 1999).
Third, regenerators that use semiconductor saturable absorbers have a speed limited by the carrier recombination rate. They also generally suppress background noise only and cannot reduce noise in the xe2x80x9c1xe2x80x9d bit or optical signal peak. Spectral noise outside of the signal spectral band is not suppressed.
What is needed are methods and apparatuses for all-optical regeneration that use a semiconductor electroabsorption modulator (SEAM).
The present invention provides a method and apparatus for all-optical regeneration that uses one or more semiconductor electroabsorption modulators (SEAMs). The SEAM exhibits nonlinear optical transmission characteristics. The SEAM is under an electrical DC reverse bias. The SEAM can be temperature-controlled. Data is transcribed from a degraded optical signal to another stabilized optical signal of the same or different wavelength.
Embodiments and applications of the present invention include an alloptical SEAM regenerator, an all-optical distributed feedback laser SEAM (DFB-SEAM) regenerator, an all-optical SEAM with fiber Bragg grating (SEAM-FBG) regenerator, an all-optical SEAM with polarizing beam splitter (SEAM-PBS) regenerator, an all-optical Mach-Zehnder interferometric (MZI-SEAM) regenerator, an all-optical 3-port MZI-SEAM regenerator, and a SEAM-based optimized receiver.
One feature of the present invention involves all-optical regeneration. The optical output signal has a wavelength identical to the optical carrier signal (also called optical data signal). A stronger non-linear transfer function is obtained by applying a high non-time-varying DC reverse bias.
An all-optical regenerator is provided that reduces the noise level on the xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d bits of the regenerated optical signal.
An all-optical regenerator is provided that increases the extinction ratio (that is, ratio of power in the xe2x80x9c1xe2x80x9d bit to the power in the xe2x80x9c0xe2x80x9d bit).
An all-optical regenerator is provided that operates at a high data rate with a simple way to adjust the response time without the requirement of expensive high-speed electronic packaging, such as, an impedance matching network.
An all-optical regenerator is provided that suppresses optical noise including in the regenerated optical signal in the temporal domain as well as spectral domain.
An all-optical regenerator is provided that has strong thresholding, is output chirp adjustable, is compact, integrable, and capable of high-speed operation. The recovery rate of the absorption can be controlled by the DC reverse bias.
An all-optical receiver and method are provided that improves sensitivity of an optical receiver through an improved decision circuit and optimization of wavelength.
Commercial off the shelf (COTS) components can be used. Example COTS can include fiber pig-tailed devices, such as, discrete electro-absorption (EA) modulators and monolithically integrated distributed feedback laser and electro-absorption (DFB-EA) modulators.
According to another feature of the present invention, optical regeneration further includes wavelength conversion. The optical output signal has a wavelength different from the optical carrier signal or optical data signal. A relatively low DC reverse bias is applied compared to optical regeneration operation.
Examples of a semiconductor electroabsorption modulator that can be used with the present invention include a semiconductor material which exhibits electroabsorption effect (such as, Franz-Keldysh effect, quantum confined Stark effect, or the Wannier-Stark effect). Examples of a SEAM that can be used with the present invention include a semiconductor material which in the form of direct bandgap bulk semiconductor material, double heterostructure layers, quantum wells, or superlattice structure. Example semiconductor materials that SEAMs can be composed of are GaAs, InGaAs, InGaAsP, InP, InGaAlAs, GaAlAs, and InAlAs.
Optically, a SEAM can exhibit low polarization sensitivity as in bulk or strained quantum wells. The absorption of the SEAM is saturable under intense optical input. Low facet reflectivity of the SEAM can be achieved with AR coatings and/or angled facets. A short length SEAM less than or equal to 200 microns can be used to reduce optical power.
Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.